Hybrid networks

ABSTRACT

The disclosure of the present specification relates to a bridge network suitable for use as a telephone hybrid wherein the bridge network includes at least two bridge networks in cascade, one bridge having arms which include the input and bidirectional ports of the hybrid and the nominal balancing impedance, the other sub-bridge network having arms which include the output port of the hybrid and the output circuit of an amplifier having an output signal characterized by a product of the signal applied to the input port of the hybrid and the signal from the output of the hybrid, the output circuit thereby being isolated from arms in the output port and from all other arms of the bridge which include reactive elements or characteristics.

United States Patent Cherry 1 51 Jan. 16, 1973 HYBRID NETWORKS 2,838,6126/1958 Pocock ..179/81 A 2,988,712 6/l96l Rhodes ..l79/l70 NC [75]Inventor l' Cherry vlctona 2,629,024 2 1953 Edwards ..179 17o NCAustralia [73] Ass1gnee: MOIIHSh UIIIVGISItLVICIOHa, Primary ExaminerRalph Blakeslee Austraha Assistant Examiner-David L. Stewart Filed: 1970Attorney-Cushman, Darby & Cushman [57] ABSTRACT The disclosure of thepresent specification relates to a Foreign Apphcatwn Priority Databridge network suitable for use. as a telephone hybrid Dec. 4, 1969Australia ..64,792/69 wherein the bridge network includes at least twoMarch 26, 1970 Australia ..PA075|/ bridge networks in cascade, onebridge having arms which include the input and bidirectional ports ofthe [52] U.S. Cl. ..l79/l70 NC, 179/81 A, 333/] l, hybrid and thenominal balancing impedance, the 333/ other sub-bridge network havingarms which include 51] Int. Cl. .......H04m l/58 the output p of thehybrid and the Output circuit of [58] Field of Search ..179/ NC, 81 A;333/11, 10; an amplifier having an Output signal Characterized y 323/75;330/ a product of the signal applied to the input port of the hybrid andthe signal from the output of the hybrid, [56] References Cited theoutput circuit thereby being isolated from arms in the output port andfrom all other arms of the bridge UNITED STATES PATENTS which includereactive elements or characteristics.

3,479,468 ll/l969 Kretzmer ..179/170 NC 17 Claims, 8 Drawing Figures3,440,367 4/1969 Holtz ..l79/8l A I g l NETWORK INPUT SGNAL 6| CONNECTEDTO j V BlDlRECTlONAL b b PORT OUTPUT PATENTEDJAH 16 I975 3.711.660

SHEET 3 [IF 3 dujarfini omziiou HYBRID NETWORKS This invention relatesto balanced" networks and more particularly, Although not exclusively,to an improved hybrid network which is capable of achieving acceptablebalance conditions.

Hybrid networks are used extensively in telephone systems and comprisethree ports: an input port, an output port and a bidirectional port. Twohybrid networks are required to set up many of the telephoneconversations between subscribers not served by the same exchange. Insuch an arrangement each subscriber is connected to the bidirectionalport of the related hybrid whilst the output port of each is connectedto the input port of the other: one circuit carries the signal .in onedirection while the other circuit carries the signal in the otherdirection.

When it is balanced, a hybrid network simultaneously satisfies thefollowing conditions:

a. a signal applied to the input port does flow out the bidirectionalport;

b. a signal applied to the input port does not flow out the output port,and

c. a signal applied to the bidirectional port does flow out the outputport.

The primary cause of imbalance in hybrid networks results from the lackof a constant relationship between the admittance of the two-wirecircuit connected to the bidirectional port and the other admittances ofthe hybrid. if the hybrid is not balanced, as is usually the case, asignal applied to the input port appears at the output port and thiswill give rise to an echo in the following manner: If the hybridconnected to one subscriber is unbalanced then a signal from a callingsubscriber is fed into the input of the unbalanced hybrid and some ofthis signal flows from the output port back to the calling subscriber sothat this subscriber hears the returned signal. Now if the call is localand the propagation time too small to produce an audible separationbetween the direct and reflected components there is no problem. In factthe echo may be used to advantage to make the phone sound alive when itis used. If the call involves a moderate propagation delay, of the orderof several tens of milliseconds, then echo becomes annoying to thesubscriber who happens to be speaking at the time. For satellitecommunication, delays of the order of several hundreds of millisecondsare involved and the echo makes conversation almost impossible.

Until now, it has been found impossible to maintain reasonable balanceof hybrids at all times and so echo suppressors are used to combat theproblem. Some suppressors act as a one-way switch and this allows onesubscriber to capture the circuit. Whilst they reduce the problem ofecho, such suppressors suffer from the practical disadvantage thatdouble-talking is prevented. Other suppressors which allow limiteddouble-talking have been proposed but these are found to be complex andexpensive.

it is the primary object of the invention to provide an improved networkin which acceptable balance conditions are achieved despite the presenceof a variable admittance in the network.

Another more specific object is the provision of a hybrid network inwhich acceptable balance conditions are achieved despite variations ofthe admittance of the circuit connected to the bidirectional port suchthat an echo which exists is reduced in amplitude to an acceptablelevel.

In one form the invention provides a bridge network in which one armincludes a first signal generator, one arm includes both a first loadimpedance and a second signal generator, one arm includes a second loadimpedance, and an arm, including any one of the preceding arms, includesthe output circuit of a device having an output signal which ischaracterized by a product of the signal from the first signal generatorand the signal across the second load.

In the first form of the invention described above, the first signalgenerator may be the transmitter portion of a telephone headset. The armincluding both'the first load impedance and the second signal generatormay be analogized to the transmission line coupled to another remotelylocated telephone hybrid set, the load representing the impedance of theline, and the signal generator representing the source of the currentwhich is transmitted over that line during a communication exchange. Thesecond load may be a resistor, or admittance (admittances are used insome of the circuit equasions further in the discussion) which iscoupled to the output circuit of the hybrid and further represents anadmittance associated with the bidirectional port. Finally the devicewhich produces an output signal which is characterized by a product ofthe signal from the first signal generator and the signal across thesecond load is multiplication device which may include a simplemultiplier circuit coupled serially with an amplifier to increase thegain thereof. The particular multiplication factor being determined ofthe balancing of the bridge network.

As will be described later, the device may also take the form of avariable attenuator. However, for purposes of explanation in the presentform of the invention it is an amplifier and multiplier circuit aspreviously noted. The output signal from the device might beproportional to the product of the input and output signals of thebridge, or the product of the moduli of these signals, or the product ofthe input and output signals exponentiated to any arbitrary power, orany combination of these alternatives.

Hereinafter the devices unless otherwise described shall be referred toas amplifier multiplier circuits or simply multiplier circuitsperforming the function of combining the signals required, amplified bythe characteristic transfer function of an amplifier.

The bridge network may take forms such as a Wheatstone bridge of sixarms (including the detector and source), or modified Wheatstone bridgessuch as the hexagonal and octahedron forms, or an interconnection ofseveral Wheatstone bridges. The well known transformer bridge or aninterconnection of several such bridges may also be used, and in eachcase the output of the multiplier may be connected across any one of theseveral node pairs of the bridge.

in another form the invention provides an improved hybrid comprising abridge network wherein one arm includes the output circuit of amultiplier having an output signal which is characterized by a productof the signal applied to the input port of the hybrid and the signalfrom the output port of the hybrid. More particularly the bridge networkincludes one arm which includes the input port, one arm which includesthe output port, and one arm which includes the bidirectional port, saidarm including said multiplier output being a separate arm or one of theabove defined arms.

It is preferred that the multiplier output be included in an arm that isisolated from the bidirectional port arm since in such an arrangementthe conductance looking into the bidirectional port can be made linearconstant. The term isolated is intended to indicate that the arm isconnected such that the multiplier output does not flow in the armincluding the bidirectional port.

It is also preferred that said multiplier output should be included inan arm that is isolated from all reactive arms of the bridge, since insuch an arrangement voltage spikes which are characteristic ofnon-linear reactive circuits can be eliminated.

Accordingly, in a third form the invention provides a bridge network inwhich one arm includes a first signal generator, one arm includes both asecond signal generator and a first load impedance, one arm includes animpedance which is nominally conjugate to the first load impedance andapproximately balances the bridge, one arm includes a second loadimpedance, and a further arm, which is isolated from said arms includingthe first load impedance and nominal balancing impedance, and whichincludes the output circuit of an multiplier having an output signalwhich is characterized by a product of the signal from the first signalgenerator and the resulting signal across the second load.

In a fourth form the invention provides an improved hybrid including abridge network comprising at least two bridge networks in cascade, onebridge having arms which include the input and bidirectional ports ofthe hybrid and the nominal balancing impedance, the other bridge networkhaving arms which include the output port of the hybrid and the outputcircuit of an multiplier having an output signal characterized by aproduct of the signal applied to the input port of the hybrid and thesignal from the output port of the hybrid.

In the third and fourth above-defined forms of the invention, said armincluding said multiplier output circuit should be isolated from saidsecond load impedance and from all reactive arms of the bridge. Suchisolation can be achieved by having the multiplier output circuit in anarm of the bridge that is conjugate to the arm including both the secondsignal generator and first load (bidirectional port), and also conjugateto the arm including the nominal balancing impedance, and by havingother appropriate arms resistive. For example, in the fourth form of theinvention, these conditions obtain if the second sub-bridge has fourappropriate arms resistive and equal.

The multiplier output signal in the second through fourth forms of theinvention may be any of the alternatives described in connection withthe first form.

In a fifth form the invention provides a bridge network in which one armincludes a first signal generator, one arm includes both a second signalgenerator and a first load impedance, one arm includes an impedancewhich is nominally conjugate to the first load impedance andapproximately balances the bridge, and a further arm which includes theinput circuit of a continuously variable attenuator with its outputconnected to a second load impedance; this attenuator has its losscontrolled by the first signal generator.

In a sixth form the invention provides an improved hybrid including abridge network, having arms which include the input and bidirectionalports of the hybrid and the nominal balancing impedance, and a furtherarm which includes the input circuit of a continuously variableattenuator with its output connected to the output port of the hybrid;this attenuator has its loss controlled by the signal applied to theinput port of the hybrid.

The bridge networks and sub-bridge networks in the second through sixthforms of the invention may take any of the forms described in connectionwith the first form.

Several preferred embodiments of the various forms of the invention willnow be described with reference to the accompanying drawings in which:

FIG. 1 is a generalized circuit diagram of the linear actively balancedbridge of Lampard and Stuart which is utilized in the bridge accordingto the invention;

FIG. 2 is a generalized circuit diagram of a hybrid employing a bridgecircuit according to the present invention;

FIG. 3 is a generalized block diagram indicating one manner of derivingthe required signal product;

FIG. 4 is a circuit diagram of an alternative selfbalancing hybrid;

FIG. 5 is a circuit diagram of one form of attenuator network which canbe used in the network of FIG. 4;

FIG. 6 is the circuit diagram of the combination of FIGS. 4 and 5;

FIG. 7 is a block diagram of a particularly preferred practicalimplementation of the invention in the form of a telephone hybrid, and

FIG. 8 is a more detailed circuit diagram of the hybrid indicatingsuitable resistance values.

In the first embodiment to be described the improved hybrid employs aWheatstone bridge of six arms. However, as is made clear above, thisshould not be construed as limiting since many other bridge networks maybe satisfactorily used.

To understand the operation of the improved hybrid, it is desirablefirst to understand the Lampard and Stuart bridge which isdiagrammatically shown in FIG. 1 Reference is made to the September 1963issue of the I.E.E.E. Transactions on Circuit Therory CT-l 0, pages 357to 362, for a full discussion of this linear actively balanced bridge,in which a balancing amplifier Al generates an output depending on thebridge output voltage v alone. That is i Y v, where Y is the transferadmittance of the amplifier Al. Consideration of this reference willshow that when signal i, is applied a signal v appears across G. Also ifY approaches infinity then by the feedback action of the amplifier thebridge output v is forced to be vanishingly small, independent of thevalue of G. Thus, if this bridge were used as a hybrid with Grepresenting the conductance of the network applied to the bidirectionalport, the above two results show that both requirements (a) and (b) of abalanced hybrid would be satisfied by this network. However, requirement(c) cannot be satisfied while requirement (b) is also satisfied since ifa signal were to be applied across G then the feedback action of theamplifier Al makes the output voltage v vanishingly small for Y verylarge. This means that a signal applied to the bidirectional port wouldnot flow out the output so the bridge would not be useful as a hybrid.

A first embodiment of the improved hybrid is diagrammaticallyrepresented by FIG. 2 and in this Figure:

Current generator or amplifier A,- produces an input signal 1,-representing the signal source connected to the input port;

Conductance G, includes the output conductance of the source;

Voltage v,, represents the signal output from the output port;

Conductance G includes any load connected to the output port;

Current generator amplifier A, produces a current output i representingthe signal applied to the bidirectional port;

Conductance G represents the conductance of the two wire networkconnected to the bidirectional port. (G is arbitrary).

Voltage v,, is the voltage appearing across the bidirectional port;

Current i,, represents the output signal of an multiplier A,-, and

Conductance G, includes the output conductance of the multiplier Theoutput signal of the multiplier A is defined by n i| 0/ T (I) where U isa constant of proportionality that relates the output of the multiplierto a product between the signals at the input and output ports of thehybrid.

It will be appreciated that, because the amplifier A, output isproportional to a product between the signals at the input and outputports, the improved hybrid shown diagramatically in FIG. 2 belongs tothe class of non-linear electrical networks.

FIG. 3 shows schematically how the signal represented by the aboveequation may be derived. The input signal i, to the hybrid is applied toa full-wave rectifier at AA' (see FIG. 2) which yields a new signal |iat BB'. This new signal and the output signal of the hybrid are thenapplied to the two input ports of a multiplier which yields the productli l v as shown. This product is then applied to an l/A3 having atransfer constant l/U so that the output signal from the amplifier is asdefined by equation 1. This amplifier A3 output and other outputsinvolving various products of input and output signals could be derivedfrom many different forms of suitable apparatus. For example it will beappreciated that in a practical arrangement the required gain would beobtained directly from the multiplier circuit and no separate amplifierneed be employed. The device enclosed within the dotted lines shown inFIG. 3 is similar to the multiplier unit 42 shown in FIG. 2 and aspreviously described since the function thereof is to provide an outputsignal which is characterized by the product of the input signal to thebridge, and the signal across the load and further amplified by thetransfer characteristic of the amplifier A3.

Referring again to FIG. 2, consideration of this network will show thatthe undesired characteristic of the Lampard and Stuart bridge preventingrequirement (c) being satisfied is removed in accordance with thisembodiment by reducing the effective transfer cmittance of the amplifierA to zero when the input signal i; is not present. Equation (1 can berearranged to express this admittance as Y (effective)=i,./v,,= li l/U(2 Thus Y (effective) is zero when i is zero.

In a practical realization of the hybrid, five of the six arms may bemade of the same conductance G with the arm defining the bidirectionalport A-B still arbitrary G. In such an arrangement it can be shown thatIt will be evident that the following results are realized:

1. Equation (3) shows that the signal v,, flowing out of thebidirectional port contains a term proportional to the input signal i,so requirement (a) of a balanced hybrid is satisfied.

2. If the output of Amplifier A (i,,) is zero, (4) reduces to Equationn= bl (6) Thus a signal applied to the A-B- flows out from the outputport, satisfying requirement (c) for a balanced hybrid.

4. For this particular configuration of the improved hybrid, the bridgehaving five equal arms and the multiplier A output in the arm conjugateB'A' to the bidirectional port A-B, Equation (3) shows that theconductance looking into the bidirectional port is linear and constantat G.

5. If neither i nor 1",, is zero, the output signal v contains the smallundesired echo term defined by Equation 5) and the desired term due toi,,, namely,

The presence of Ii in the denominator shows that the desired term due toi,, is intermodulated by input signal i that is, it is distorted.However, the zero crossings of the above term are the zero crossings ofi,,. It is wellknown that a speech waveform, even if severely distorted,remains intelligible if its zero crossings are unchanged. Therefore, theoutput signal from the selfbalancing hybrid is intelligible anddouble-talking is possible. It will be appreciated that theintermodulation distortion is increased by increasing the gain of themultiplier circuit (reduction of U U should therefore be chosen only assmall as is necessary to achieve acceptable balance and thus echosuppression.

All these results hold, independent of the admittance of G connected tothe bidirectional port A-B. The general forms of results l (2), (3) and(5) hold if the five arms are not of equal conductance so that all threerequirements of a balanced hybrid are still satisfied.

Tests have been carried out using a system embodying the generalizedfeatures of the hybrid network shown in FIG. 2 and it was found that theamplitude of the echo was reduced by factors of twenty and higher suchthat the affect on conversations involving large propagation times wasfound to be negligible.

In the preferred embodiment of the invention described above, all armsof the bridge are conductive. When the load presented to thebidirectional port is a complex admittance of nominal value Y and actualvalue Y, then the conjugate arm of the bridge would be changed to G /Yin order to restore approximate balance of echo. If the multiplieroutput A is included in one arm of the bridge, large voltage spikesappear at the output port and the unwanted echo can actually beincreased rather than reduced; these spikes are due to circulatingcurrents which are changing the stored energy and are a characteristicof circuits that are both non-linear and reactive.

The hybrid circuit of FIG. 4 differs from the hybrid of FIG. 2 in thatthe multiplier Al is omitted and that the output signal v,, is derivedfrom a voltage attenuator V connected to the detector terminals of thebridge. The attenuator V, has an input admittance of G and the ratio ofoutput voltage to input voltage [v /v is 1 represented by l/(l lid/Jwhere 1 is a transfer constant of the attenuator. The equationsdescribing FIGS. 2 and 4 are similar in form.

In this arrangement the bridge network is linear'so that, even if theload impedance connected to the bidirectional port is reactive, theabove mentioned voltage spikes do not appear.

Many suitable forms of variable attenuator could be I devised by personsskilled in the art. Examples are a thermistor network or active bridgenetwork for which the loss is continuously variable, and anelectricallyswitched resistor network for which the loss is variable ina number of discrete but closely-spaced steps and approximates to beingcontinuously variable.

One preferred form of attenuator is shown in FIG. 5 to comprise abalanced Wheatstone bridge including the output circuit ofa multiplier Ain the arm normally occupied by the detector. The multiplier output is nl Il "tr/ r where U, is a transfer constant of the multiplier A Voltagesv, and v, are derived from the indicated arms of the bridge.

The combined circuit is shown in FIG. 6 and it is to be appreciated thatthere is shown a cascaded pair of Wheatstones bridges B, constituting abridge circuit itself. Since the Bridge B constitutes an attenuator thisembodiment falls within the scope of the first, fifth and sixth formdisclosed. Furthermore, since the pair of bridges constitutes a bridgecircuit itself, this embodiment would also fall within the scope of thefirst and second forms disclosed. It will be evident that this bridgecircuit B -B, the pair produces similar equations to those described inFIG. 2, so that the bridge is selfbalancing to the same extent as thesimple bridge disclosed above. For example bridge B may be analogized toattenuator 8,, shown in FIG. 4 or output port 8-8 of FIG. 2. This is notto say they are identical but merely that they serve a similar purposethat is isolation of the input from the output. In this case however, anonlinear multiplier output signal does not appear at the port includingLoad L of the output resistive bridge B a (as amplifier A2 in FIG. 2)multiplier output A,,,, which is connected to the reactive bridge 8,contains the bidirectional port and the nominal balancing impedance G G/y, said non-linear output of the multiplier A,, is isolated from saidreactive bridge B,-. Accordingly, since neither of the bridges issimultaneously non-linear and reactive, the voltage spikes discussedabove are not present This embodiment of the invention therefore fallswithin the scope of the third and fourth form disclosed.

The circuit shown in FIG. 4 may be thought to bear some resemblance tothe well known echo suppressor. However, the following featuresdistinguish it quite clearly:

I. In the self-balancing hybrid herein the controlled attenuator V. inthe output path is continuously variable; in the echo suppressor it isan on-off switch.

11. In the self-balancing hybrid the control signal to the attenuator V,is the instantaneous input signal Ii in the echo suppressor it is thetime average of |i,| taken over several periods of the speech waveform.

In combination these two differences make doubletalking" possible withthe self-balancing hybrid, where double-talking" is not possible withthe echo suppressor.

In some circumstances it may be advantageous to use the time average of[i l as one input to the multiplier of the embodiments of FIGS. 2 and 6or as the signal to the attenuator of FIG. 4. At the zero crossings ofthe i,- waveform, the multiplier A4 output is zero and the attenuator V,is set to minimum loss. In either case there is no echo suppression atall. When all admittances in the hybrid are pure conductance, the zerocrossings of i,- coincide with the zero crossings of the echo v,,. Thus,echo suppression ceases only at instants when there is no echo tosuppress. However, when complex admittances are included in the hybrid,the zero-crossings of i, may not coincide with the zero-crossings of vand echo can become objectionable.

If a time average of |i,| is used as the control signal and theaveraging period is of the order of one speech waveform period, themultiplier A output not zero and the attenuator V is not set to minimumloss at the zerocrossings of i and the objectionable echo is reduced.The price paid is some degradation of the performance under double-talkconditions.

In a still further form the invention there is provided a network inaccordance with any one of the forms of the invention defined abovewherein a control signal lid which is an input multiplier A orattenuator is time averaged over a short period, for example, of theorder of one speech waveform.

One way of introducing this time averaging would be by means of a filter(such as a shunt capacitor) at a full wave output of the rectifier (notshown) included in amplifier A7 which is used to derive [i,| from iAnother method might be to use some device with an inherent time lag asa variable arm of the bridge or attenuator for example, a thermistor.

it is known to use time averaged signals in echo suppressors but theperiod in such instances is usually more than ten speech waveformperiods.

FIG. 7 shows a block diagram of a practical implementation of theinvention in the form of a telephone hybrid. This implementation isbased on FlG. 6 which falls within the scope of all six disclosed formsof the invention. Admittances 6,; G G G /Y| and Y corresponds to Fourarms of the bridge 8, in FIG. 6 and the four conductances G, correspondto the Four arms of the bridge 8,. The following additional elementssimplify the design of a practical hybrid:

l. A buffer amplifier A, is provided at the input port, for supplyingthe current drive i, and for partially making up the inherent loss ofthe network.

2. A buffer amplifier A, is provided at the output port, for making upthe remaining loss of the network.

3. A ground point is provided shown symbolically, so

that signal voltages at the ports are specified relative to grounddatum.

If it is not desired that voltages should have ground datum,differential amplifiers might be used, or balanced-to-unbalancedtransformer might be included at the ports. Alternatively, the wholenetwork might be realized as a transformer bridge.

it is convenient to set all conductive arms of the bridge equal to'G:

where G is the nominal value of the admittance connected to thebidirectional port at some frequency near the middle of thevoice-frequency band; 0.0012 mho would be a suitable value. it is alsoconvenient to set the gains of the input and output amplifiers as l/ in4 Dlll l) 1 for the loss in the hybrid is then zero for the special caseof l Y G. The equations for the hybrid are:

when the multiplier output is [n the detailed design of the inputamplifier and multiplier circuits use could be made of the fact that ad.c. supply constitutes a signal ground. The quiescent currents for theamplifier can therefore be supplied in part via the arms of the bridge.

FIG. 8 shows in outline suitable circuits:

The amplifier and multiplier A may be separate as described in FIG. 3,however, for cases for ease of manufacture the combined form is usuallymore convenicnt. The multiplier in FIG. 3 has two inputs, one for thefull wave rectifier and the other coupled to the output voltage V,,.Similarly in FIG. 8 full wave rectifier R, drives the bases of atransistors T and T while B is amplified and multiplied by transistors Tand T As was suggested above in relation to the embodiment of FlG. 2,the value of the multiplier A, transfer constant must be a compromise.For a telephone hybrid in which signal peaks at the bidirectional port 5are of the order of l V, a suitable balancing amplifier gain is of theorder of U =0.l V.

The circuit in FIG. 8 is shown to provide an example of component valueswhich might be used to implement the present invention.

While there has been described, what at present is considered to be thepreferred embodiment of the present invention, it will be obvious tothose skilled in the art, that various changes and modifications may bemade herein, without departing from the invention, and it is thereforeaimed in the appended claims, to cover all such changes andmodifications as fall within the true spirit and scope of the invention.

l claim:

1. A bridge network in which a first arm includes a first signalgenerator, a second arm includes both a first load impedance and asecond signal generator, a third arm includes a second load impedance,and a fourth arm includes a third signal generator having an outputsignal which is characterized by a product between the signal from saidfirst signal generator and the signal across said second load impedance.

2. The bridge network according to claim 1, wherein said third signalgenerator includes a multiplying circuit for obtaining said signalproduct.

3. An improved hybrid comprising a bridge network according to claim 2,wherein said first arm includes the input port, said second arm includesa bidirectional port, and said third arm includes the output port.

4. The improved hybrid according to claim 3, wherein the impedance ofone arm of said bridge network is so chosen as to approximately balancethe hybrid.

5. The improved hybrid of claim 3 wherein said third signal generator isconnected in an arm of said bridge network and wherein said bridgenetwork is balanced such that the output signal from said third signalgenerator does not flow in said bidirectional port.

6. The improved hybrid according to claim 3 wherein said third signalgenerator is connected in an arm of said bridge network and wherein saidbridge network is balanced such that the output from said third signalgenerator does not flow in any arm in which the peak energy stored isgreater than one tenth of the energy dissipated per cycle at a signalfrequency of the first signal generator.

7. The hybrid network according to claim 3 wherein the output of saidthird signal generator is defined by i =1, v,,/U wherein i, is thesignal presented to the input port, v, represents the signal of theoutput port and U is the transfer constant of said multiplying circuit;said bridge circuit comprising a Wheatstone bridge of six arms, suchthat the voltage signal at the bidirectional port is represented by theexpression:

wherein i,, is the signal applied to the bidirectional port, G is theconductance of five of the six arms and G is the arbitrary conductanceof the bidirectional port; the voltage signal at the output port beingrepresented by the expression:

8. The improved hybrid according to claim 7 wherein the bridge networkcomprises at least two bridge networks coupled from an input of one toan output of the other, one bridge network having arms which include theinput and bidirectional ports'of the hybrid and the balancing impedance,the other bridge network having arms which include the output port. ofthe hybrid and said third signal generator.

9. The improved hybrid of claim 8 wherein said third signal generator isconnected in an arm of said bridge network for which said'bridge networkis so balanced that the output signal from said third signal generatordoes not flow in said bidirectional port.

10. The improved hybrid according to claim 8 wherein said third signalgenerator is connected in an arm of said bridge network for which saidbridge net-' work is so balanced that the output signal from said thirdsignal generator does not flow in any arm in which the peak energystored is greater than one tenth of the energy dissipated per cycle atthe signal frequency of the first signal generator.

11. The improved hybrid according to claim 8 wherein the other bridgehas four appropriate arms constituted by resistive elements and isbalanced so that the output signal of said third signal generator doesnot flow in the bidirectional port or in any arm in which the peakenergy stored is greater than one tenth of the energy dissipated percycle at the signal frequency of factor controlled by the signal fromsaid first signal generator.

13. An improved hybrid comprising a bridge network according to claim 12wherein said arm includes the input port, said second arm includes thebidirectional,

port, and said second load impedance includes the impedance connected tothe output port.

14. The improved hybrid according to claim 13 wherein the impedance ofone arm of the bridge network is so chosen as to approximately balancethe hybrid.

15. The improved hybrid according to claim 13 wherein the ratio ofoutput voltage to input voltage of the attenuator is represented by l/H-[i l /J wherein J is a transfer constant of the attenuator and i is thesignal presented to the input port.

16. The bridge network according .to claim 2 wherein an input signal tothe amplifier circuit is time averaged over a period of the order of onespeech waveform.

17. The bridge network according to claim 12 wherein the control signalto the attenuator is time averaged over a period of the order of onespeech waveform.

1. A bridge network in which a first arm includes a first signalgenerator, a second arm includes both a first load impedance and asecond signal generator, a third arm includes a second load impedance,and a fourth arm includes a third signal generator having an outputsignal which is characterized by a product between the signal from saidfirst signal generator and the signal across said second load impedance.2. The bridge network according to claim 1, wherein said third signalgenerator includes a multiplying circuit for obtaining said signalproduct.
 3. An improved hybrid comprising a bridge network according toclaim 2, wherein said first arm includes the input port, said second armincludes a bidirectional port, and said third arm includes the outputport.
 4. The improved hybrid according to claim 3, wherein the impedanceof one arm of said bridge network is so chosen as to approximatelybalance the hybrid.
 5. The improved hybrid of claim 3 wherein said thirdsignal generator is connected in an arm of said bridge network andwherein said bridge network is balanced such that the output signal fromsaid third signal generator does not flow in said bidirectional port. 6.The improved hybrid according to claim 3 wherein said third signalgenerator is connected in an arm of said bridge network and wherein saidbridge network is balanced such that the output from said third signalgenerator does not flow in any arm in which the peak energy stored isgreater than one tenth of the energy dissipated per cycle at a signalfrequency of the first signal generator.
 7. The hybrid network accordingto claim 3 wherein the output of said third signal generator is definedby ia ii vo/UT wherein ii is the signal presented to the input port, vorepresents the signal of the output port and UT is the transfer constantof said multiplying circuit; said bridge circuit comprising a Wheatstonebridge of six arms, such that the voltage signal at the bidirectionalport is represented by the expression: ii ( 1/2(G+G'') ) + ib ( 1/G+G'') wherein ib is the signal applied to the bidirectional port, G is theconductance of five of the six arms and G'' is the arbitrary conductanceof the bidirectional port; the voltage signal at the output port beingrepresented by the expression: ( ii/G+ ii /4UT ) ( G-G''/8(G+G'') ) + (ib/G+ ii /4UT ) ( G/2(G+G'') )
 8. The improved hybrid according to claim7 wherein the bridge network comprises at least two bridge networkscoupled from an input of one to an output of the other, one bridgenetwork having arms which include the input and bidirectional ports ofthe hybrid and the balancing impedance, the other bridge network havingarms which include the output port of the hybrid and said third signalgenerator.
 9. The improved hybrid of claim 8 wherein said third signalgenerator is connected in an arm of said bridge network for which saidbridge network is so balanced that the output signal from said thirdsignal generator does not flow in said bidirectional port.
 10. Theimproved hybrid according to claim 8 wherein said third signal generatoris connected in an arm of said bridge network for which said bridgenetwork is so balanced that the output signal from said third signalgenerator does not flow in any arm in which the peak energy stored isgreater than one tenth of the energy dissipated per cycle at the signalfrequency of the first signal generator.
 11. The improved hybridaccording to claim 8 wherein the other bridge has four appropriate armsconstituted by resistive elements and is balanced so that the outputsignal of said third signal generator does not flow in the bidirectionalport or in any arm in which the peak energy stored is greater than onetenth of the energy dissipated per cycle at the signal frequency of thefirst signal generator.
 12. A bridge network in which a first armincludes a first signal generator, a second arm includes both a firstload impedance and a second signal generator, and a third arm includesthe input circuit of a continuously variable attenuator having itsoutput connected to a second load impedance, said attenuator having itsloss factor controlled by the signal from said first signal generator.13. An improved hybrid comprising a bridge network according to claim 12wherein said arm includes the input port, said second arm includes thebidirectional port, and said second load impedance includes theimpedance connected to the output port.
 14. The improved hybridaccording to claim 13 wherein the impedance of one arm of the bridgenetwork is so chosen as to approximately balance the hybrid.
 15. Theimproved hybrid according to claim 13 wherein the ratio of outputvoltage to input voltage of the attenuator is represented by 1/1+ ii /JTwherein JT is a transfer constant of the attenuator and ii is the signalpresented to the input port.
 16. The bridge network according to claim 2wherein an input signal to the amplifier circuit is time averaged over aperiod of the order of one speech waveform.
 17. The bridge networkaccording to claim 12 wherein the control signal to the attenuator istime averaged over a period of the order of one speech waveform.